This invention relates to a control circuit for gas discharge lamps, and more particularly relates to a control circuit which permits improved dimming of large numbers of various kinds of gas discharge lamps.
The present invention is an improvement over the circuits disclosed in U.S. Pat. No. 4,350,935, dated Sept. 21, 1982, entitled "Gas Discharge Lamp Control" in the name of Joel S. Spira et al. and assigned to the assignee of the present invention. As disclosed in U.S. Pat. No. 4,350,935, it is possible to regulate the output light of one or more fluorescent lamps by applying a voltage wave form to the lamp ballast which has a notch in each of the half waves, which notch is of variable width and of variable location within the half wave form.
The circuit arrangement shown in U.S. Pat. No. 4,350,935 provides good operation over a wide range but has several shortcomings. For example, the circuit employs a series switching means and shunt switching means for an inductive ballast. The series switching means is a high speed transistor which is operable to turn off at some desired point in the input voltage wave form to produce the desired notch in the input voltage. The shunt switching means turns on during this notch interval to provide a bypass path for the discharge of energy from the ballast. The shunt switching devices consist of anti-parallel connected controlled rectifiers. If, for any reason, a spurious control signal is applied to the controlled rectifiers out of their proper sequence, it becomes possible to produce a short circuit from the a.c. voltage power line through the series switching transistor and the parallel switching device. This could seriously damage or destroy the series switch.
Another shortcoming of the circuit of U.S. Pat. No. 4,350,935 is that the lamp life of energy saving lamps reduces when the lamps are operated in their lower dimming end region. One reason for this is that as the notch width increases, the RMS content of the voltage applied to the inductive ballast decreases. As a consequence, the effective output voltage of the filament transformers decreases so that the lamps will extinguish at relatively low dimming.
A further difficulty experienced with the arrangement of U.S. Pat. No. 4,350,935 is "tracking" several banks of lamps so that they dim by the same amount. Proper tracking requires placement of the notch close to the start of each of the half waves in the nearly fully illuminated condition so that the notch can move to the right during dimming without causing some or all of the lamps to drop out while remaining lamps become very bright.